Diesel engine

ABSTRACT

A diesel engine includes a variable swirl forming mechanism, an exhaust gas recirculation (EGR) control mechanism, and an injection timing control mechanism. The variable swirl forming mechanism forms strong, middle or weak air swirls in a combustion chamber according to engine loads and engine speeds. The EGR control mechanism recirculates a part of exhaust gas to the combustion chamber according to the engine loads and the engine speeds. The injection timing control mechanism advances or retards the injection timing according to the engine loads and the engine speeds.

This application is a division of copending application Ser. No.07/835,545, filed on Feb. 14, 1992, now U.S. Pat. No. 5,186,139. Theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a diesel engine which assures excellent engineperformance and can purify exhaust gas satisfactorily.

2. Description of the Related Art

The diesel engine has to meet very strict requirements for much cleanerexhaust gas. A wide assortment of measures have been proposed anddevised for this purpose. Various control systems have been proposed andput into practical use to decrease nitrogen oxides (NOx) in the exhaustgas, and hydrocarbon in particulates and soot, respectively.

A variable swirl control system has been proposed to form air swirls ina combustion chamber, for thereby efficiently forming an air-fuelmixture to burn the fuel completely and decrease the particulate andsoot. An exhaust gas recirculation (EGR) control system recirculatespart of exhaust gas to the combustion chamber according to the workingconditions of the engine, for decreasing the concentration of oxide,lowering the combustion temperature, and suppressing NOx. A timingcontrol system controls the fuel injection timing to reduce NOx andparticulates. Furthermore, efforts have been made to devise shapes offuel injection units and combustion chambers. The exhaust gas dischargedvia the foregoing control systems are further filtered by an exhaust gaspurifier located in a scavenge passage.

Any of the foregoing control systems however cannot independently copewith all the problems of the exhaust gas. These control systems haveadvantages and disadvantages. To purify the exhaust gas by a singlecontrol system, such a control system inevitably becomes morecomplicated and more expensive. Specifically, it is not possible at allto have a single control system purify the exhaust gas of a vehicleengine whose working condition changes incessantly. Conventional controlsystems are considered acceptable when they purify the exhaust gas to acertain preferable degree. Usually, to emit cleaner exhaust gas, engineperformance is somewhat sacrificed. For instance, suppression of NOxresults in the increase of concentration of smoke and hydrocarbon, andthe decrease of the engine efficiency. Therefore, the foregoing controlsystems make it difficult to meet the new requirements for much cleanerexhaust gas so as to prevent air contamination.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a diesel enginewhich can emit a much cleaner exhaust gas without decreasing the engineperformance.

According to the invention, there is provided a diesel enginecomprising: a variable swirl forming mechanism for generating strong airswirls in a combustion chamber when the engine is working at low speedsand high loads, middle air swirls in the combustion chamber when theengine is working at low loads and at middle speeds and high loads, andweak air swirls in the combustion chamber when the engine is working athigh speeds and high loads; an exhaust gas recirculation (EGR) controlmechanism for recirculating part of the exhaust gas to the combustionchamber when the engine is working at low and middle speeds and lowloads; an injection timing control mechanism for advancing the fuelinjection timing when the engine is working at low loads, and forretarding the fuel injection timing when the engine is working at middleand high loads; and a controller for controlling the variable swirlforming mechanism, EGR control mechanism and injection timing controlmechanism according to the loads and speed of the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a block diagram showing the configuration of a diesel engineaccording to an embodiment of the invention;

FIG. 2 is a side cross sectional view of a variable swirl formingmechanism;

FIG. 3 is a side cross sectional view of the variable swirl formingmechanism of FIG. 2;

FIG. 4 is an axial cross sectional view of a fuel injection pump;

FIGS. 5a to 5e show the operation sequence illustrating how fuel isdelivered under pressure by a plunger of the fuel injection pump;

FIG. 6 is a diagram showing one example of a swirl control map;

FIG. 7 is a diagram showing one example of an EGR control map;

FIG. 8 is a diagram showing one example of an injection advance map;

FIG. 9 is a graph showing the relationship between an EGR ratio and theload;

FIG. 10 is a graph showing the relationship between the EGR ratio andthe engine speed;

FIG. 11 is a graph showing the relationship between the injectionadvance timing and the load;

FIG. 12 is a flowchart of a main routine for controlling the exhaustgas;

FIG. 13 is a flowchart of a program for variable swirl control;

FIG. 14 is a flowchart of a program for EGR control; and

FIG. 15 is a flowchart of a program for injection advance timing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an embodiment of the invention, a diesel engine includes avariable swirl forming mechanism, an exhaust gas recirculation (EGR)control mechanism, an injection timing control mechanism, and acontroller 9, as shown in FIG. 1.

The variable swirl forming mechanism forms strong, middle or weak airswirls in a combustion chamber according to engine speeds and engineloads, and includes a main port 5a, a sub-port 5b, an auxiliary variableswirl valve 5c, and a drive unit for operating the valve 5c.

The EGR control mechanism recirculates part of the exhaust gas from ascavenge passage 11 to the combustion chamber 1 according to engineconditions. This control mechanism includes an EGR passage 12, an EGRvalve 13, and a passage selecting valve 14.

According to engine loads and speeds, the controller 9 controls thevariable swirl forming mechanics, the EGR control mechanism and theinjection timing control mechanism.

As shown in FIG. 1, a diesel engine includes combustion chambers, a fuelinjection pump, a variable swirl forming mechanism for forming weak,middle and strong air swirls in a combustion chamber, an exhaust gasrecirculation (hereinafter called "EGR") control mechanism, an injectiontiming control mechanism, and a controller for controlling the foregoingvariable swirl forming mechanism, the EGR control mechanism and theinjection timing control mechanism.

Each combustion chamber 1 includes a cylinder 2, a piston 3 and acylinder head 4. The cylinder head 4 has an inlet valve 6 for openingand closing an inlet port 5, and an injection nozzle 7 for injectingfuel at a proper timing. The injection nozzle 7 is connected to the fuelinjection pump 8.

The fuel injection pump 8 is controlled by the timing control mechanismof the controller 9, and the fuel injection pump 8, operated to jet thefuel through the injection nozzle 7 at a proper timing depending uponthe engine speed Ne and the load L.

The inlet port 5 communicates with an inlet passage 10 connected to anair cleaner (not shown). The inlet port 5 includes a main port 5a havinga large cross-sectional area, and a sub-port 5b having a smallcross-sectional area.

The sub-port 5b is opened and closed by an auxiliary variable swirlvalve 5c. The auxiliary variable swirl valve 5c is operated by thecontrol signal from the controller 9 depending upon the engine speed Neand the load L. When the auxiliary variable swirl valve 5c fully closesthe sub-port 5b, air is drawn only through the main port 5a, and strongair swirls are formed in the combustion chamber 1. When the auxiliaryvariable swirl valve 5c fully opens the sub-port 5b, the air is drawnvia both the main port 5a and sub-port 5b, and weak air swirls areformed in the combustion chamber 1. When the sub-port 5b is half closed,middle air swirls will be formed in the combustion chamber 1. Themovements of the auxiliary variable swirl valve 5c are controlled by adriving unit such as a solenoid-operated valve and a motor which is setin motion according to the signal from the controller 9.

The variable swirl forming mechanism includes a plurality of inlet units24, inlet manifolds 40 and driving units, which are provided forrespective combustion chambers. The variable swirl forming mechanism iscontrolled by the controller 9.

As shown in FIGS. 2 and 3, each inlet unit 24 is located in the cylinderhead 4. Each intake manifold 40 includes a main branch pipe 41, anauxiliary branch 42, and an air chamber 43. The main branch pipe 41communicates with the combustion chamber 1 via the main port 5a. Themain branch pipe 41 has a large cross-sectional area. The auxiliarybranch pipe 42 communicates with the sub-port 5b, which has an opening28a near the inlet valve 6 and an inlet valve seat 26. The opening 28aof the sub-port 5b faces a circumferential wall of a swirl chamber 25awhich is located at the end of the main port 5a.

The auxiliary variable swirl valve 5c is rotatable to open and close thesub-ports 5b of all the intake manifolds 40, and is located at one endeach of each sub-port 5b. The auxiliary variable swirl valve 5c includesa shaft 5d, of which an outer surface is cut into a shape similar to aplurality of butterfly valves. The shaft 5d is slightly thicker thaneach auxiliary branch pipe 42. Hereinafter, a description will be madeby taking one of the intake manifolds 40 as an example. The valveauxiliary variable swirl 5c is angularly moved to take three positionsto control the airflow. Specifically, when the auxiliary variable swirlvalve 5c is perpendicular to the airflow, the auxiliary variable swirlvalve 5c closes the auxiliary branch pipe 42 completely. When theauxiliary variable swirl valve 5c is in parallel to the airflow, theauxiliary variable swirl valve 5c fully opens the branch pipe 42. Whenthe auxiliary variable swirl valve 5c is half tilted to the airflow, theauxiliary variable swirl valve 5 c half opens the branch pipe 42.

As shown in FIG. 2, the shaft 5d is longitudinally inserted into theintake manifold 40 via a hole 42a which is perpendicular to theauxiliary branch pipe 42, from the right side in FIG. 2. The cut portion5e of the shaft 5d confronts the auxiliary branch pipe 42. The left endof the shaft 5d projects from the intake manifold 40, and ismechanically coupled to a drive mechanism 31, which is a motor.Reference numeral 32 stands for an angle sensor for detecting theangular movement of the shaft 5d.

The drive mechanism 31 is electrically connected to the controller 9,which is a microcomputer.

When the valve auxiliary variable swirl 5c fully closes the sub-port 5b,the inlet valve 6 is descended to move the inlet valve seat 26downwardly in the inlet stroke, for thereby conducting the air, which iseccentric in the main port 5a, into the combustion chamber 1, andforcibly forming strong and large air swirls in the combustionchamber 1. This air is mixed with the fuel from the injection nozzle 7(shown in FIG. 1) so as to be burned.

The controller 9 sends an operation signal to the drive mechanism 31 inresponse to engine speeds and engine loads. The controller 9 keeps onoperating the drive mechanism 31 according to the angular movement ofthe shaft 5d detected by the angle sensor 32. When the shaft 5d is movedto open the auxiliary variable swirl valve 5c fully, a large amount ofthe air is introduced into the sub-port 5b via the auxiliary branch pipe42, and is mixed with the main air (which is conducted via the main port5a in the forward swirl), and supplied to the combustion chamber 1.Under this condition, the air from the sub-port 5b reverses the mainair, which weakens the main forward air swirls.

When it is found that middle air swirls are sufficient to meet thepresent engine speed and the engine load, the controller 9 operates thedrive mechanism 31 to half open the auxiliary variable swirl valve 5c.The amount of the air via the sub-port 5b is decreased to weaken thebackward air swirls and to form middle air swirls in the combustionchamber 1.

The fuel injection pump 8 will be described in detail by referring toFIG. 4.

A housing 52 holds therein a plurality of barrels 54 which are axiallyjuxtaposed on one side of the housing 52. Each barrel 54 has a firstbarrel member 54a and a second barrel member 54b fitted into the firstbarrel member 54a.

A delivery valve holder 56 is connected to a cylinder located atop thebarrel 54, and has a delivery valve 57a fitted therein.

A plunger 58 is slidably fitted in the barrel 54, and is downwardlyurged by a spring 60. A cam 62 is coupled to a drive shaft of theengine, not shown, to push the plunger 58 upwards.

A control sleeve 64 is slidably fitted around the plunger 58. Thesliding movement of the control sleeve 64 is regulated by a guide pin 66which is fitted into a guide groove of the control sleeve 64.

A joint 68 is movably supported by the second barrel member 54b and isfixedly secured to the plunger 58.

The plunger 58 has an oil gallery 58a, an opening 58b, a slit 58c, and aslit 58d. The oil gallery 58a communicates with the upper end andcircumferential surface of the plunger 58. The opening 58b is formed onthe surface of plunger 58, and communicates with the oil gallery 58a.The slit 58c is axially formed on the surface of the plunger 58. Theslit 58d is formed on the plunger, and is tilted with respect to theaxis of the plunger 58. The slits 58c, 58d and the opening 58b serve asa control groove. The control sleeve 64 has a through-hole 64a forcontrolling the end of the fuel injection stroke.

A fuel chamber 65 stores the fuel supplied from a fuel feed pump, whichis not shown. This fuel does not leak into a camshaft chamber 63 sincethe plunger 58 is oil-tightly fitted in the second barrel member 54b inthe cylindrical shape.

A guide pin 73 projects from a tappet 75, and is slidably fitted into aguide groove 77 formed on the housing 52.

An adjusting shaft 76 is for fine adjustment of the fuel injectiontiming, and has an adjusting screw 79 threadably fitted in a screw holethereof. The adjusting shaft 76 is turned by loosening the adjustingscrew 79 for the foregoing purpose.

When a cam 62 is rotated once by a camshaft 62a which is operated by thedrive shaft of the engine, a tappet roller 75a reciprocates the plunger58 in one stroke each time the tappet roller 75 is pushed by the cam 62.In other words, the aforementioned reciprocation of the plunge deliversthe fuel under pressure.

The reciprocation sequence of the plunger 58 is described hereinafter byreferring to FIGS. 5a to 5e. It is assumed that the control sleeve 64 isat its home position during the sequence shown in FIGS. 5a and 5b. Whenthe control sleeve 64 and plunger 58 are relatively positioned as shownin FIG. 5a, i.e. the opening 58b is not yet fully closed by the controlsleeve 64, a pressurized chamber 70 communicates with the fuel chamber65, so that no fuel is delivered.

When the opening 58b is closed by the control sleeve 64 as shown in FIG.5c following the state shown in FIG. 5b, the pressurized chamber 70 isinsulated from the fuel chamber 65, and is pressurized by the plunger58. The plunger 58 is lifted to the position of FIG. 5c from theposition of FIG. 5a. This movement of the plunger 58 is called"prestroke."

The plunger 58 keeps on moving upwards as shown in FIG. 5d. Then, thepressure in the pressurized chamber 70 overcomes the force of a spring57 of the delivery valve holder 56, for opening an output port 57a, andsupplying the pressurized fuel to the injection nozzle 7 via aninjection pipe 56a (shown in FIG. 4). The pressurized fuel is deliveredto the injection nozzle 8 until the slit 58d communicates with thecontrol hole 64a as shown in FIG. 5e. When the slit 58d begins to reachthe control hole 64a, the pressurized chamber 70 communicates with thefuel chamber 65 via the oil gallery 58a, the opening 58b and the slit58c, for thereby completing delivery of the pressurized fuel.

As the plunger 58 is moved upwardly by the joint 68 (shown in FIG. 4),the timing for the slit 58d to reach the control sleeve 64 can bechanged during the stroke of the plunger 58. The amount of the fuel tobe injected per stroke of the plunger 58 can be adjusted accordingly.The movement of the joint 68 is controlled by longitudinally displacinga rack 74 engaged with a ball 72 which is fixedly secured to the top ofthe joint 68.

The injection timing control mechanism will be described hereinafter.The injection timing is controlled by sliding the control sleeve 64around the plunger 58. The control sleeve 64 is slid by the adjustingshaft 76, a lever 78, and a cut 64b. The adjusting shaft 76 is locatednear the control sleeve 64, and has an axis which is parallel to theplane where the foregoing barrels 54 are juxtaposed and which isperpendicular to the axis of the plunger 58. The lever 78 is fixedlysecured to the adjusting shaft 76, and projects toward the plunger 58.The cut 64b is formed on the outer surface of the control sleeve 64, forreceiving one end of the adjusting shaft 76. With this arrangement, theadjusting shaft 76 moves to slide the control sleeve 64 via the cut 64b.

The control sleeve 64 is displaced downwardly or upwardly to advance orretard the fuel injection timing. The outer end surface of the lever 78is rounded to be always in close contact with the inner surface of thecut 64b.

In response to a signal from the controller 9, the adjusting shaft 76 isoperated by an electromagnetic solenoid via an operation lever connectedto the other end of the adjusting lever 76. The electromagnetic solenoidand the operation lever are not shown in FIG. 4. The control sequence ofthe fuel injection timing will be described later.

Referring to FIG. 1 again, the EGR control mechanism is described here.An exhaust gas recirculation passage 12 (called "EGR passage 12"hereinafter) is branched from the scavenge passage 11, and is connectedto the inlet passage 10 via an EGR valve 13. The EGR valve 13 is forrecirculating part of the exhaust gas to the combustion chamber 1, andis actuated by a passage selecting solenoid valve 14, a duty solenoidvalve 15 and a vacuum pump 16, all of which are actuated according tothe working condition of the engine.

The duty solenoid valve 15 undergoes the duty control in response to asignal from the controller 9, for supplying a negative pressure to theEGR valve 13 from the vacuum pump 16 via the passage selecting solenoidvalve 14. The passage selecting solenoid valve 14 opens and closes theEGR valve 13 in response to the EGR control signal from the controller9, for controlling recirculation of the exhaust gas.

The controller 9 operates as described hereinafter. The controller 9 issupplied with an engine speed signal from an engine speed sensor (Nesensor), and an engine load signal from a load sensor (L sensor) whichdetects the engine load according to the operated extent of anaccelerator.

The controller 9 has a swirl control map for operating the variableswirl forming mechanism at a proper timing. In FIG. 6, the ordinaterepresents the load L, and the abscissa represents the engine speed Ne.The strong, middle and weak swirl regions are mapped as A, B, and C,respectively, in FIG. 6. In FIG. 6, L max stands for the maximum load,Ne max stands for the maximum engine speed. Lo=(0.6 to 0.7) L max, Ne₁=(0.45 to 0.55) Ne max, and Ne₂ =(0.65 to 0.75) Ne max.

The strong swirl region A corresponds to the engine working range at lowspeeds and high loads, in which the engine speed Ne<Ne₁ and the loadL≧L₀. In the strong swirl region A, the auxiliary variable swirl valve5c closes the sub-port 5b to introduce the air into the combustionchamber 1 only via the main port 5a, for forming the strong air swirl inthe combustion chamber 1 to enhance the formation of the air-fuelmixture.

The weak air swirl region C corresponds to the engine working range athigh engine speeds and high loads, in which Ne≧Ne₂ and L≧L₀. Theauxiliary variable swirl valve 5c fully opens the sub-port 5b to conductthe air into the combustion chamber 1, for thereby forming weak airswirls therein.

The middle air swirl range B corresponds to the engine operating rangeat low-loads, and high loads and middle speeds. The sub-port 5b is halfopened to form middle air swirls.

As shown in FIG. 7, the EGR control map of the controller 9 includes theEGR region D in which the auxiliary EGR valve 13 is opened to circulatepart of the exhaust gas, and the non-EGR region E where the EGR valve 13is closed. The EGR region D corresponds to low loads in which L₁ isequal to or less than 0.45 to 0.55 times L max, and low and middleengine speeds in which Ne₃ is equal to or less than 0.8 to 0.85 times Nemax.

The operation of the fuel injection pump 8 is controlled according tothe injection advance control map (which has the characteristic as shownin FIG. 8) so as to inject the fuel via the fuel injection nozzle 7.When the engine speed Ne=Ne*, the injection timing θinj is advanced orretarded according to the load as shown in FIG. 11. The injection timingis extensively advanced when the load L is extremely low. On thecontrary, the injection timing is retarded for the middle and high loadranges.

The main routine for controlling the exhaust gas will be described withreference to FIG. 12. When an ignition key is turned on to start theengine, all the data are initialized in the controller 9. The controller9 reads various data on the engine speed Ne and the engine load L. Thecontroller 9 checks whether or not the engine speed Ne and the load Lare normal. If not, control returns to the step to the engine speedcheck Ne and the load L. When the engine speed Ne and the load L areabnormal, the controller 9 turns on an alarm lamp to warn maloperationof the engine speed sensor or the load sensor.

Referring to FIG. 13, the variable swirl control routine will bedescribed. When the engine is periodically operated by the timer, thecontroller 9 checks the present working condition of the engine based onthe read engine speed Ne and the load L with reference to the variableswirl control map (shown in FIG. 6).

When the engine is working at low speeds and high loads, fuel is jettedin the shape of large particles into the combustion chamber 1 from thefuel injection nozzle 7, and a little air is conducted into thecombustion chamber 1, so that incompletely burned fuel tends to form assoot. Therefore, if the engine speed Ne is smaller than Ne₁ and the loadL is larger than L₀, the controller 9 recognizes that the strong airswirl region A should be established, and operates the drive mechanism31, which angularly moves the auxiliary variable swirl valve 5csubstantially close to the sub-port 5b. Then, air is introduced only viathe main port 5a (shown in FIG. 5) so as to form the strong air swirlsin the combustion chamber 1. The strong air swirls are mixed with thefuel injected at the specified timing, for thereby enhancing formationof air-fuel mixture and complete combustion of the fuel and, suppressingformation of soot.

When the engine is working at high speeds and high loads, the fuel isinjected in the form of fine particles and a large amount of air isconducted into the combustion chamber 1. At the engine speed Ne>Ne₂ andthe load L>L₀, the controller 9 recognizes that the weak air swirlregion C should be established. Then, the controller 9 fully opens theauxiliary variable swirl valve 5c to conduct a large amount of air viathe main port 5a and sub-port 5b, for thereby forming weak air swirls inthe combustion chamber 1 and suppressing soot.

In the swirl control map shown in FIG. 6, the region B corresponds tothe engine speed Ne and the load L as follows: Ne<Ne₁ and L<L₀, Ne₁≦Ne<Ne₂, and Ne>Ne₂ and L<L₀. When the load L is low in the region B,the amount of the fuel to be injected is small and a relatively largeamount of air is introduced regardless of the engine speed. Since thestrong air swirl is not necessary in the region B, so that thecontroller 9 operates the drive mechanism 31 to angularly move theauxiliary variable swirl valve 5c, for thereby half opening the sub-port5b. Thus, the combustion temperature is prevented from becomingunnecessarily high, for thereby decreasing hydrocarbon.

At middle engine speeds and high engine loads, the injection pressure ofthe injection nozzle 7 is high enough to jet the fuel in the form ofrelatively small particles. Furthermore, the amount of the conducted airis moderate, so that the middle air swirls are formed to suppresshydrocarbon and soot.

The EGR control mechanism operates according to the sequence shown inFIG. 14. When the engine is started by the timer at the specifiedintervals, the controller 9 checks the engine speed Ne and the load Lread therein, and identifies the present working condition of the enginereferring to the EGR control map (FIG. 7). When the engine is working atloads L≦L₁ and speeds Ne≦Ne₃ which correspond to the EGR range D, thecontroller 9 operates the duty solenoid valve 15 and passage selectingvalve 14 (shown in FIG. 1) so as to transmit the negative pressure tothe EGR valve 13 from the vacuum pump 16, open the EGR passage 12, andconduct part of the exhaust gas to the inlet passage 12. The EGR range Dis established when the engine is working at low loads and low andmiddle speeds, so that the exhaust gas is recirculated to decrease theamount of fresh air to be introduced and to reduce NOx by lowering thecombustion temperature. In this case, L₁ =(0.45 to 0.55) L max, and Ne₃=(0.8 to 0.85) Ne max.

FIG. 9 shows the relationship between the EGR ratio and the load L whenthe engine speed Ne is Ne** (shown in FIG. 7). The controller 9 controlsthe duty solenoid valve 15 so that a high EGR ratio is maintained forlow loads L* and is gradually lowered as the load L reaches thethreshold value L₁. Thus, the controller 9 regulates the amount of theexhaust gas to be recirculated. As the load L becomes higher, more airis necessary for fuel combustion. In this case, no exhaust gas isrecirculated, for thereby suppressing soot.

FIG. 10 shows the relationship between the EGR ratio and the enginespeed Ne when the load L is L* (shown in FIG. 7). The amount of theexhaust gas to be recirculated is decreased to conduct as much fresh airas possible when the engine speed Ne is extremely low and when Ne₃ isapproximately (0.8 to 0.85) Ne max.

When the engine is working at speeds and loads corresponding to thenon-EGR region D (shown in FIG. 7), the controller 9 closes the EGRvalve 13 so as not to recirculate the exhaust gas. This is because muchsoot would be formed if exhaust gas were recirculated under thiscondition.

The injection advance timing will be described with reference to FIG.15. When the engine is started by the timer at the specified intervals,the controller 9 checks the engine speed Ne and the load L storedtherein, and determines the injection advance timing according to theinjection advance control the map. As shown in map of FIG. 8, as theengine speed.Ne becomes higher, the injection timing is advanced tolengthen the combustion stroke and operate the engine efficiently at anyload. The injection timing θ inj is retarded from the timing θ inj* whenthe engine is working at the speed Ne=Ne* and at middle and high loadsas shown in FIG. 11. When the load L is extremely low, the injectingtiming θ inj is extensively advanced to expediate the fuel injection.

When the engine is working at higher loads, more NOx will be emitted. Insuch a case, the fuel injection timing is retarded to decrease formationof NOx. When the engine loads are not so high, the combustiontemperature is relatively low, so that NOx will not be formed in a largeamount. As the loads become lower, less fuel will be injected. In thiscase, the fuel is jetted in the form of large particles, which willcause incomplete burning of the fuel, and the increase of particulatescontaining hydrocarbon. To cope with this phenomenon, the fuel injectiontiming is extensively advanced at very low loads to lengthen the periodduring which the air and fuel are mixed to enhance complete burning ofthe fuel. Therefore, as small amount of fuel will remain unburned, andhydrocarbon and SOF in the particulates will be decreased.

Needless to say, the foregoing variable swirl control, EGR control andfuel injection timing control are simultaneously carried out dependingon the engine working conditions.

According to the invention, the diesel engine can decrease NOx in theexhaust gas, and particulates containing hydrocarbon to a satisfactorydegree by a combination of the variable swirl control mechanism, the EGRcontrol mechanism and the injection timing control mechanism. Thesecontrol mechanisms function in cooperation with each other so as toprevent the decrease of the engine efficiency and to keep excellentengine performance.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for controlling a diesel engine,comprising the steps of:(a) generating strong air swirls in a combustionchamber when the engine is working at low speeds and high loads, middleair swirls in said combustion chamber when the engine is working at lowloads and at middle speeds and high loads, and weak air swirls in saidcombustion chamber when the engine is working at high speeds and highloads by a variable swirl forming mechanism; (b) recirculating a part ofexhaust gas to said combustion chamber when the engine is working at lowand middle speeds and low loads by an exhaust gas recirculation (EGR)control mechanism; (c) advancing the fuel injection timing when theengine is working at low loads by an injection timing control mechanism;(d) retarding the fuel injection timing when the engine is working atmiddle and high loads by said injection timing control mechanism; and(e) controlling said steps (a)-(d) according to loads and speeds of theengine by a controller.
 2. A method for controlling a diesel engineaccording to claim 1, wherein said controller closes an auxiliary swirlvalve of said variable swirl forming mechanism at said step (e) so as toconduct air only via a main port of said variable swirl formingmechanism and to form strong air swirls in said combustion chamber whenengine loads are equal to or more than 0.6 to 0.7 times the maximum loadand engine speeds are equal to or less than 0.45 to 0.55 times themaximum speed.
 3. A method for controlling a diesel engine according toclaim 1, wherein said controller opens an auxiliary swirl valve of saidvariable swirl forming mechanism at said step (e) so as to conduct airvia a main port and a sub-port of said variable swirl forming mechanismand to form weak air swirls in said combustion chamber when the engineloads are equal to or more than 0.6 to 0.7 times the maximum load andengine speeds are equal to or more than 0.65 to 0.75 times the maximumspeed.
 4. A method for controlling a diesel engine according to claim 1,wherein said controller half closes an auxiliary swirl valve of saidvariable swirl forming mechanism at said step (e) so as to conduct airvia a main port and a sub-port of said variable swirl forming mechanismand to form middle air swirls in said combustion chamber when engineloads are equal to or less than 0.6 to 0.7 times the maximum load and atany engine speed, and when engine speeds are more than 0.45 to 0.55times the maximum speed and equal to or less than 0.65 to 0.75 times themaximum speed and when engine loads are equal to or more than 0.6 to 0.7times the maximum load.
 5. A method for controlling a diesel engineaccording to claim 1, wherein said controller opens a valve of said EGRcontrol mechanism at said step (e) to establish an EGR region whereexhaust gas is recirculated to said combustion chamber when engine loadsare equal to or less than 0.45 to 0.55 times the maximum load and enginespeeds are equal to or less than 0.8 to 0.85 times the maximum speed. 6.A method for controlling a diesel engine according to claim 5, whereinsaid controller controls said valve of said EGR control mechanism atsaid step (e) so as to decrease the amount of the exhaust gas to berecirculated when the engine is working at approximately 0.45 to 0.55times the maximum load in said EGR region.
 7. A method for controlling adiesel engine according to claim 6, wherein said controller controlssaid valve of said EGR control mechanism at said step (e) to decreasethe amount of exhaust gas to be recirculated when engine speeds are lowin said EGR region or when engine speeds are approximately 0.8 to 0.85times the maximum speed, and to increase the amount of exhaust gas to berecirculated when engine speed are between the foregoing ranges.
 8. Amethod for controlling a diesel engine according to claim 4, whereinsaid controller opens said valve of said EGR control mechanism at saidstep (e) so as to establish an EGR region where exhaust gas isrecirculated to said combustion chamber when engine loads are equal toor less than 0.45 to 0.55 times the maximum load and engine speeds areequal to or less than 0.8 to 0.85 times the maximum speed.
 9. A methodfor controlling a diesel engine according to claim 8, wherein saidcontroller controls said valve of said EGR control mechanism at saidstep (e) so as to decrease the amount of the exhaust gas to berecirculated when the engine is working at approximately 0.45 to 0.55times the maximum load is said EGR region.
 10. A method for controllinga diesel engine according to claim 9, wherein said controller controlssaid valve of said EGR control mechanism at said step (3) so as todecrease the amount of exhaust gas to be recirculated when engine speedsare low in said EGR region or when engine speeds are approximately 0.8to 0.85 times the maximum speed, and to increase the amount of exhaustgas to be recirculated when engine speeds are between the foregoingranges.
 11. A method for controlling a diesel engine according to claim1, wherein said controller controls said injection timing controller atsaid step (e) to retard the injection timing at low engine speeds,linearly advance the injection timing according to an increase of enginespeeds, and maintain a predetermined injection timing at high enginespeeds.
 12. A method for controlling a diesel engine according to claim11, wherein said controller advances the injection timing at low engineloads, retards the injection ing most at middle engine loads, andadvances the injection timing gradually at high engine loads at saidstep (e).
 13. A method for controlling a diesel engine according toclaim 10, wherein said controller retards the injection timing at lowengine speeds, linearly advances the injection timing according to anincrease of engine speeds, and maintains a predetermined injectiontiming at high engine speeds at said step (e).
 14. A method forcontrolling diesel engine according to claim 13, wherein said controlleradvances the injection timing at low engine loads, retards the injectiontiming most at middle engine loads, and advances the injection timinggradually at high engine loads at said step (e).